This invention relates to optical devices.
More particularly the invention relates to optical devices suitable for use in coupling a planar optical waveguide formed on a substrate to a waveguide off the substrate, for example, an optical fibre.
Planar optical waveguides fabricated on a substrate in semiconductor material usually have relatively large differences between the refractive indices of the materials forming the waveguide. As a result, strong optical waveguiding is obtained even in structures with relatively thin layers. Indeed if the layers are too thick the waveguides will support more than one guided mode.
The relatively strong waveguiding obtained with thin layers is convenient from a fabrication point of view since growth of very thick structures is avoided. However, a problem arises when light is to be coupled from the waveguide into an optical fibre. The strong guiding in the semiconductor typically leads to spotsize radii of the order of 0.5 to 1.0 .mu.m in contrast to that of fibres which are typically in the range 3 to 6 .mu.m. This leads to tight assembly tolerances and inefficient coupling or necessitates the use of complicated matching optics.
If the wave is al)owed to diverge from the end of the substrate waveguide the spot size can be matched, but the phases of the outer parts of the wave will cause them to cancel the central part and reduce the coupling efficiency drastically.
To overcome this problem in planes parallel to the plane of the substrate, tapering of the waveguide on the substrate can relatively easily be provided. However, tapering in the direction transverse to the plane of the substrate is much more difficult, see for example an article entitled "Tapered waveguide InGa/InGaAsP multiple quantum-well lasers" by V. Koren et al at page 136 of the proceedings of the Integrated Photonics Research Conference, 1989.